DESCRIPTION: (Adapted from Applicant's description) First full term pregnancy during the teen age years in women can permanently reduce their risk for developing breast cancer. In the current proposal, the investigator will thoroughly analyze the causes of parity-related refractoriness to mammary cancer development in rats, and examine the mechanism underlying the protection effect of perphenazine, a drug which causes transient prolactinemia. The proposal includes four Specific Aims: [1] Studies of the refractoriness of parous rats to MNU-induced mammary carcinogenesis, [2] whether the refractoriness is permanent, [3] characterize the perphenazine-induced refractoriness, and [4] the mechanism underlying the perphenazine-induced refractoriness. The long-term goal is to generate an experimental paradigm that can be used to develop strategies for the prevention of human breast cancer without significant alterations in the quality of life.